Sensors with implantable or insertable electrode systems facilitate measurements of physiologically significant analytes such as, for example, lactate or glucose in an individual's body. The working electrodes of systems of this type have electrically conductive enzyme layers in which enzyme molecules are bound and release charge carriers by catalytic conversion of the analyte. In the process, an electrical current is generated as a measuring signal whose amplitude correlates to the analyte's concentration. Such electrode systems are disclosed in, for example, Int'l Patent Application Publication Nos. WO 2007/147475 and WO 2010/028708.
The working electrodes of the electrode system typically are provided with a diffusion barrier to control diffusion of the analyte to be determined from the body fluid or tissue surrounding the electrode system to the enzyme molecules that are immobilized in the enzyme layer. According to Int'l Patent Application No. WO 2010/028708, the diffusion barrier of the electrode system is a solid solution of at least two different polymers, preferably acrylates. The polymers may be copolymers such as, for example, copolymers of methyl methacrylate and hydroxyethyl methacrylate or copolymers of butyl methacrylate and hydroxyethyl methacrylate.
Likewise, Int'l Patent Application Publication No. WO 2007/147475 discloses a diffusion barrier made from a polymer having a zwitterionic structure. An example of such a polymer is poly(2-methacryloyloxyethyl phosphorylcholine-co-n-butylmethacrylate). The zwitterionic polymer may be mixed with another polymer such as, for example, polyurethane.
The use of polymer or copolymer mixtures, however, has drawbacks in that the preparation of the mixture and its application to the sensor is tedious and potentially problematic. Usually, the polymers to be mixed are individually dissolved and the resulting solutions are thereafter mixed in the desired ratio. This may result in precipitation of one of the components and consequently in workability problems (e.g., in a spraying process). Increased difficulties occur when the mixture includes a polymer with ionic characteristics (i.e., when one of the polymers to be mixed includes a monomer having anionic or cationic groups). The presence of such charged groups has a strong effect on the solubility, making it difficult to find a solvent suitable for both the charged polymer and an uncharged polymer.
Int'l Patent Application Publication No. WO 2006/058779 discloses an enzyme-based sensor with a combined diffusion and enzyme layer including at least one polymer material, and particles carry an enzyme, where the particles are dispersed in the at least one polymer material. The polymer may be hydrophilic as well as hydrophobic polymer chain sequences such as, for example, the polymer may be a high or low water uptake polyether-polyurethane copolymer. The use of block copolymers having at least one hydrophilic block and at least one hydrophobic block as a diffusion layer is not disclosed.
EP Patent Application Publication No. 2163190 discloses an electrode system for measuring an analyte concentration in vivo that includes a counter electrode with an electric conductor and a working electrode with an electric conductor on which an enzyme layer having immobilized enzyme molecules is localized. A diffusion barrier controls diffusion of the analyte from surrounding body fluids to the immobilized enzyme molecules. The diffusion barrier includes hydrophilized polyurethanes obtained by polycondensation of 4,4′-methylene-bis-(cyclohexylisocyanate) and diol mixtures that may be polyethyleneglycol and polypropyleneglycol. The hydrophilic polyurethane layer may be covered with a spacer such as, for example, a copolymer of butyl methacrylate and 2-methacryloyloxyethyl-phosphoryl choline. The use of block copolymers having at least one hydrophilic block and at least one hydrophobic block as a diffusion layer is not disclosed.
For the foregoing reasons, there is a need for electrode systems having improved diffusion barriers/cover layers.